The present invention relates to the production of polyamides by the anionic polymerization of lactams.
Polyamides, particularly polylactams which are a class of polyamides prepared by polymerizing lactam monomers, exhibit a wide range of properties suitable for a variety of applications. Polycaprolactam, such as that available as NYLON 6, is perhaps the most widely used polyamide because it exhibits excellent mechanical and physical properties at a moderate cost.
Polycaprolactam can be polymerized under substantially anhydrous conditions by anionic catalytic polymerization. That is, the process is initiated with a strong base. Such suitable catalysts for the polymerization are lactam anions which can be obtained from lactam-metal compounds having a metal atom, typically an alkali or alkaline earth metal, bound to their nitrogen atom. For example, sodium caprolactamate, potassium caprolactamate, and Grignard compounds such as caprolactam magnesium bromide can be utilized.
The anionic polymerization process can be appreciably accelerated by the addition of an initiator or promoter to the polymerization reactants. For example G.B. No. 1,099,265; G.B. No. 1,067,153; and U.S. Pat. No. 3,308,099 describe the addition of an initiator to accelerate polymerization. The promoters or initiators which have been suggested by the art include, for example, compounds containing a tertiary nitrogen atom to which are bound two or more carbonyl, thio carbonyl, sulfonyl or nitroso groups; compounds which react with a lactam monomer to form one of these aforelisted compounds, for example isocyanates, acid chlorides, or acid anhydrides; and compounds such as bislactams, for example, terephthaloyl bis caprolactam. Polymerization is generally carried out by using a single catalyst in conjunction with a single type of initiator.
Uses of polycaprolactam include the production of fibers, fabrics, as well as molded and laminated articles. For some applications a combination of properties is desirable, for example, impact resistance, stiffness, heat sag resistance, rapid cure and reduced water absorption; hence polymers of polycaprolactam plasticized with various polymeric polyols are generally prepared to suit particular demands. The preparation of these plasticized polymers, however, is not without attendant difficulties. Often, certain desirable properties are diminished, such as rapid cure with a given level of catalyst and initiator. Moreover, other difficulties can arise, for example, the presence of high levels of unreacted lactam monomer can give rise to a malodorous finished product.
Over the years efforts have been directed toward the goals of making the polymerization of lactams a more efficient process and at the same time achieving a multiplicity of desirable properties in the finished product. A simple, economical, and expeditious manner of achieving these goals is needed.
In accordance with the present invention, there is provided a method of molding which comprises:
(I) depositing in a mold a charge of a polymerizable composition, comprising lactam monomer, initiator, and catalyst; PA0 (II) polymerizing the charge while in the mold so as to form an at least partially cured article; PA0 (III) removing the article from the mold; wherein the improvement comprises utilizing as the initiator and catalyst components a Grignard catalyst for lactam polymerization and an initiator system, comprising:
(a) at least one material represented by the following structural formula: ##STR1## (b) at least one material represented by the following structural formula: ##STR2## wherein q and v are independently an integer of at least 1; Z and W are independently selected from a compound of polymeric material, said compound being composed of at least two elements in chemical combination which contain up to two repeating structural units, said polymeric material being a macromolecule formed by the chemical union of at least two coreactants and having at least three repeating structural units; with the proviso that at least a portion of either or both the materials (a) or (b) is a polymeric material; R, R', R", R'" are independently hydrogen, a C.sub.1 to C.sub.10 aliphatic radical, C.sub.3 to C.sub.10 cycloaliphatic radical, C.sub.6 to C.sub.10 aryl or alkaryl radical, and independently R and R" and the carbon to which each is attached are included as part of a cycloaliphatic or aromatic ring; and Y is an alkylene group having from 3 to 12 carbon atoms or an alkylene group having from 3 to 12 carbon atoms interrupted by a heteroatom; with the proviso that the equivalent ratio ##STR3## groups to the magnesium of the Grignard catalyst is within the range of from 0.3 to 1 to 4 to 1, and at least 10 percent of the initiator groups of the initiator system are of the (a) type.
Also provided in accordance with the present invention is a polyamide which has been prepared by anionically polymerizing a lactam monomer in the presence of a Grignard catalyst and an initiator system as has been set forth above. The polyamide can be processed into, for example, fibrous or pellet form for later use, for example, one component injection molding.